Invasion of Seawater-Derived Fluids at Very High Temperatures in the Oman Ophiolite – a Key for Cooling the Deep Crust at Fast-Spreading Ridges

Abstract:

Prominent conceptual models for the formation of the deep, fast-spread crust are the "gabbro-glacier" model, where the lower crust is formed in the axial melt lens, and the "sheeted sill" model, where the lower gabbros are generated by the intrusion of sills of gabbroic mushes. A requirement for the latter model is a substantial hydrothermal cooling of the oceanic crust in the depth, and as long as the "Rosetta stone" for the mechanism of this deep cooling is not found, the "sheeted sill" model and derivatives cannot be accepted as reliable option how the deep oceanic fast-spread crust is formed.

In recent field campaigns for establishing a geochemical and petrological profile through typical fast-spreading oceanic crust in the Wadi Gideah (Wadi Tayin massif, Oman ophiolite), we discovered several, often more than 100 m wide fault zones, cutting a coherent series of layered gabbro at many places. These zones are characterized by pervasive alteration, mainly in greenschist and sub-greenschist facies. Isotope geochemical studies of these zones imply that these zones can be interpreted as pathways for channeled hydrothermal flux, in accord with observations of Coogan et al. (2006). In most of the fault zones, we observed the occurrence of varitextured hornblende gabbro, alternating with zones of former layered gabbros showing intense overgrowth of high- temperature amphibole, and sometimes with flasered amphibolites. The petrologic record implies fluid flux in the center of this zones at very high temperatures (≤ 1000°C), enabling even the production of hornblende gabbros by hydrous partial melting of layered gabbro, shielded by zones formed at high-temperature conditions (formation of high-T amphiboles) and at low-temperature conditions (greenschist facies rocks). First thermal modeling based on the petrological record of these zones are in progress.

Coogan LA, Howard KA, Gillis KM, Bickle MJ, Chapman H, Boyce AJ, Jenkin GRT, Wilson RN (2006) Am. J. Sci. 306: 389-427